1. Field of the Invention
The present invention relates to a photomagnetic recording apparatus which is provided with a magnetic field-generating device for generating a magnetic field required for the recording and/or the erasing of information.
2. Description of the Related Art
FIG. 1 is a view schematically illustrating a conventional photomagnetic recording apparatus. A magnetic recording medium 21 employed in this apparatus has a magnetic film which is formed thereon for example by use of deposition and has magnetic anisotropy in a direction perpendicular to the surface thereof. The magnetic film is initially magnetized in one direction perpendicular to the surface of the magnetic recording medium. The direction of this magnetization is reversed if the magnetic film is irradiated with an optical beam by an optical pickup 20 while being simultaneous subjected to, (by a magnetic field-generating device 19), a magnetic field whose direction is opposite to that of the initial magnetization of the magnetic film. The recording of information is carried out by reversing the direction of the magnetization in accordance with the information. The information, thus recorded, can be erased by irradiating the magnetic film with an optical beam while simultaneously applying to the film a magnetic field whose direction is the same as that of the initial magnetization.
Published Unexamined Japanese Patent Applications No. 57-40761 and No. 59-119507 disclose examples of the magnetic field-generating device which can be employed in the above apparatus.
FIG. 2 shows the example disclosed in Published Unexamined Japanese Patent Application No. 59-119507; and is an enlarged view partially illustrating a magnetic field-generating device 19 of the photomagnetic recording apparatus shown in FIG. 1. Referring to FIG. 2, magnetic field-generating device 19 comprises a magnetic circuit made up of a main magnetic pole 11 and a yoke 18, and coil 15 formed around the magnetic circuit. To prevent a magnetic field from decreasing in intensity at the end of main magnetic pole 11, length L of main pole 11 is set to be greater than the irradiation range A (FIG. 1) of an optical beam so that the main pole 11 covers the irradiation range A.
In the case of the example shown in FIG. 2, coil 15 is provided throughout the whole length L of main magnetic pole 11. With this construction, the series resistance of coil 15 becomes higher in proportion to the number of turns of coil 15, so that coil 15 generates a large amount of heat.
In addition, that portion of main magnetic pole 11 around which coil 15 is provided can have a rectangular or an oval cross section. With this construction, it is difficult to obtain a regularly-wound coil. Since such a coil is formed automatically by machines in the usual case, the cross section of the portion around which coil 15 is formed should be as circular as possible. If the cross section is circular, the coil can be wound with uniform tension, thus enabling the coil to be formed very regularly. If the cross section is rectangular or oval, however, the coil cannot be formed with uniform tension, so that the resultant coil is likely to become loose at the portion indicated by .alpha. in FIG. 2.
FIG. 3 shows the example disclosed in Published Unexamined Japanese Patent Application No. 57-40761. The magnetic field-generating device of this example is obtained by dividing the main magnetic pole 11 shown in FIG. 2 into a plurality of magnetic pole elements 11a, 11b, and 11c. These magnetic pole elements are linearly arranged, and one insulated wire 15 is wound around them in a series fashion. Length L indicated in FIG. 3 is determined such that it substantially corresponds to the irradiation range A of an optical beam.
In the case of the example shown in FIG. 3, the magnetic pole is divided into a plurality of elements and these elements are arranged in the direction in which an optical beam is irradiated. With this construction, it is impossible to obtain a uniform magnetic field distribution as can be seen from the graph shown in FIG. 4. The magnetic field distribution in FIG. 4 is depicted as having a sinusoidal wave form throughout length L corresponding to the irradiation range of an optical beam.
Let it be assumed that the minimum magnetic field required for the recording or erasing of information with respect to magnetic recording medium is denoted by Hmin. In this case, the magnetic field intensity at the lowest point in L of the FIG. 4 graph should be Hmin or more. Therefore, the magnetic field intensities at the other points are considerably higher than Hmin. As a result, a large amount of current is consumed in vain in the case of the example shown in FIG. 3.